From the Facebook feed of the Canadian Heraldic Authority:
Today marks the 250th birthday of King William IV, who reigned from 1830 to 1837. Our illustration of his arms comes from a Canadian source, the Proclamation of the Lieutenant-Governor of Upper Canada announcing a reward for the apprehension of William Lyon Mackenzie, the leader of the unsuccessful rebellion in Toronto in 1837. The document was issued in the name of the new monarch, Queen Victoria, yet it still used William IV’s royal arms. Can you spot the difference?
King William died on June 20, 1837; William Lyon Mackenzie’s rebellion took place in October, November, and December of that year. The Proclamation for his capture is dated December 7.
The difference, of course, is the inescutcheon at the fess point, consisting of the Hanoverian arms (of Brunswick, Lüneburg, and Westphalia). Victoria, being a woman, could not inherit these territories, so they went to her uncle, Ernest Augustus I. The Hanoverian arms were then removed, leaving the British Royal Arms in the form they are found today (1 & 4 England, 2 Scotland, 3 Ireland).
I like the nineteenth-century custom of showing the supporters leaping out from behind the shield.